Microphthalmia-associated transcription factor (MITF) plays an important role in regulating development, differentiation and function of melanocytes. MITF has a basic-helix-loop-helix-leucine zipper (bHLHZip) structure (Hodgkinson C A et al, 1993, Cell, 744: 395-404). The MITF gene in humans is located on the third chromosome 3P14.1-3P12.3 in humans. Studies have shown that it plays a critical role in regulating development, differentiation and function of pigment cells. Mutation of the MITF gene results in developmental deficit and functional impairment of pigment cells, as well as complex interactions between MITF and other signaling molecules.
MITF can regulate expression of a gene family for tyrosine, and thus participate in the regulation of melanogenesis. (Shibahara, S et al, Pigment Cell Res, 1998, 11: 329-336). The tyrosinase gene family has three members: tyrosinase, tyrosinase related protein-1 (TRP-1), and tyrosinase related protein-2 (TRP-2). Promoters of the genes all contain an “M box” structure with a “CATGTG” core, and MITF can bind to the structure to transactivate expression of the respective genes. By interaction with the promoters of the tyrosinase gene family, MITF, on the one hand, directs specific expression of the genes in melanocytes and, on the other hand, is involved in regulating melanogenesis in melanocytes by external stimuli. It has been found from the analysis of deletion and mutation in human tyrosine genes that, tyrosine promoters contain three structures: TDE, M box and E box. The three structures all contain a “CATGTG” core motif and are necessary for effective expression of tyrosine in pigment cells, and promoters with the same structures only leads to weak expression in non-pigment cells. Further studies have found that MITF can bind to these structures to stimulate transcriptional activity of the promoters. MITF itself is cell-specific and plays a critical role in specific expression of tyrosinase in melanocytes. TRP-1 and TRP-2 also have the M-box structure which is highly conserved among the tyrosine family, and MITF can bind to the M-box in the TRP-1 gene to activate its expression and direct its specific expression in melanocytes (Aksan I et al, Mol Cell Biol, 1998, 18(12): 6930-8).
Melanocytes are one of important constituents of skin cells, are of embryonic neural crest origin, have dendritic protrusions, and belong to gland cells. The synthesized melanin is secreted into keratinocytes via the dendritic protrusions, and is discharged from the body with shedding of keratinocytes and is present on the basal layer of the epidermis. Melanocytes give the skin its color through the synthesis of melanin, and also can absorb ultraviolet radiation to protect the body from being injured by ultraviolet radiation. Mammalian skin and hair colors are mainly determined by relative amount and distribution of melanin produced by melanocytes (Sturm R A et al, Bioessays, 1998, 20: 712). Melanin is further divided into Eu-melanin (brown/black) and Pheo-melanin (red/yellow) (Newton J M et al, Mamma Genome, 2000, 11: 24). The regulation of development, differentiation and melanin synthesis of melanocytes is a complex process involving in many signaling molecules to create a complex signaling network in which MITF plays an important role.
RNA interference (RNAi) is a biological phenomenon which is ancient and highly conserved in evolution, which is a post-transcriptional gene silencing in which efficient specific degradation of homologous mRNA is induced by double-stranded RNA (dsRNA) (Andrew Fire et al, Nature, 1998, 391: 806-811). Small interfering RNAs (siRNAs) are effector molecules of RNAi, which are made up of two complementary RNA strands and have 21-23 nucleotides (nt) in length. Because the RNAi technology can specifically knock out or shut down expression of a particular gene, the technology has been widely used in various studies of functional genomics, infectious diseases and antiviral and antitumor therapies. Some studies have reported that by inhibiting the MITF gene by RNAi modulation, downstream tyrosinase (TYR) gene and tyrosinase related protein-1 (TRP-1) gene (Busca B et al, J Cell Biol, 2005, 49) can be regulated, thereby regulating the growth of melanin.